Such hybrid vehicles are disclosed in, for example, Japanese Laid-Open Patent Publication Nos. H6(1994)-319210, H9(1997)-322307, H10(1998)-73161, 2003-205767, 2004-84679 and 2004-210123. As described in these disclosures, various types of hybrid vehicle have been proposed. For example, the following ideas are incorporated in a system where the vehicle is driven by the driving force being transmitted to drive wheels from both the engine and the motor generator or selectively from one or the other, i.e., a system where the engine and the motor generator are arranged in series. When the vehicle is started from standstill or accelerated, the driving force is supplied from both the engine and the motor generator for ensuring an accelerating power. On the other hand, when the vehicle is decelerated (with the accelerator being released), the operation of the engine is controlled to terminate (in idling-elimination control), or the suction and exhaust valves of some or all of the cylinders are controlled to close in partial or all cylinder-off control for improving fuel efficiency and for reducing emission. In addition, the force from the side of the drive wheels is transmitted to the motor generator for energy regeneration and for braking action, which assists the engine brake.
Generally, the driving force from the engine and the motor generator is transmitted not directly to the drive wheels but through a transmission that varies the gear ratio or speed change ratio, and an automatic transmission that comprises a torque converter and a ratio-change mechanism is often used as such a transmission. In a hybrid vehicle equipped with such an automatic transmission, while the vehicle is being decelerated (with the accelerator being released) with the force from the drive wheels being transmitted to the motor generator, slips are likely to occur in the torque converter, resulting in a low transmission efficiency. This condition leads to problems of low efficiency in energy regeneration, low effectiveness in engine brake assistance and the like.
In the case where the engine is stopped for energy regeneration while the vehicle is being decelerated, the lock-up clutch of the torque converter is engaged in the automatic transmission not to allow slippage in the torque converter, for improving the energy regeneration efficiency.
Furthermore, for example, the above mentioned Japanese Laid-Open Patent Publication No. 2004-210123 discloses a hybrid vehicle, which solves the above mentioned problems. In this hybrid vehicle, when the vehicle is being decelerated (with the accelerator being released), the operation of the motor generator is controlled to reduce the slippage of the torque converter (in cooperative control of the motor generator) for bring the lock-up clutch quickly into engagement. Additionally, the control system disclosed in the above mentioned Japanese Laid-Open Patent Publication No. 2004-210123 starts the operation of the engine with its cylinders turned off. In this case, the quick engagement of the lock-up clutch enables a prompt transition to the operation of the engine with its cylinders off, effecting an improvement in fuel efficiency.
On the other hand, while the vehicle is being decelerated, generally the speed of the vehicle decreases, and in response to this slowdown, the automatic transmission is controlled to downshift. However, if a shifting is executed while the lock-up clutch is engaged for improving the energy regeneration efficiency, then there can be a sudden change in the braking force (so-called engine brake force), which may give a sense of incongruity to the driver. For this problem, Patent Reference 3 discloses a system that allows no shifting while energy regeneration is performed during deceleration of the vehicle.
By the way, if the operation of the motor generator is controlled cooperatively, then the lock-up clutch can be engaged promptly. However, at the time of the start of the deceleration, when the accelerator pedal is released from its held down position, the slip ratio of the lock-up clutch is often relatively small. Therefore, in many cases, the lock-up clutch can be engaged quickly and smoothly without cooperative operation of the motor generator. In such cases, if the motor generator is operated in cooperation, then the driving torque of the motor generator in cooperation can be transmitted to the drive wheels, causing a fluctuation in the driving torque (accelerating torque). This fluctuation is a problem that may give the driver a sense of incongruity in the operation of the vehicle.
Moreover, there is a case where it is difficult to reduce the slip ratio of the torque converter by the cooperative operation control of the motor generator. There is also a case where the slip ratio of the torque converter does not decrease even though the cooperative operation control is executed, or even a case where the slip ratio of the torque converter increases contrary to what is expected from the execution of the cooperative operation control. In such cases, the continuation of the cooperative operation of the motor generator results only in a waste of electrical power, which leads to a problem of depletion of the battery charge.
Further, in a case where no shifting is allowed while the vehicle is in deceleration and where, for example, the deceleration of the vehicle is effected by the termination of the engine operation caused by a fuel cutoff while the transmission is set at a high speed ratio, the rotational speed of the engine can become lower than an idling rotational speed. In such a case, it is necessary to cancel the fuel cutoff and to restart the operation of the engine for accelerating the vehicle again. This situation results in a problem that lowers the fuel efficiency. In addition, it is necessary for the vehicle to downshift for the re-acceleration because the vehicle coasting at a relatively low speed with its transmission set at a high speed ratio cannot gain a drive power necessary for acceleration. This presents a problem of reduced drivability.
While it is desirable to shift the speed ratio in response to changes in the operating condition of the vehicle, it is necessary to prevent shift shock that may occur from the shifting and give a sense of incongruity. Therefore, it is proposed that the motor generator be driven during the shifting to bring the rotational speed on the side of the engine (i.e., the rotational speed of the input shaft of the transmission) closer to that expected for the speed change ratio after the shifting (refer to, for example, Japanese Laid-Open Patent Publication No. H6(1994)-319210). This control is hereinafter referred to as cooperative operation control of the motor generator for the shifting.
However, if the torque of the motor generator is controlled during the shifting mainly for bringing the rotational speed on the side of the engine to a rotational speed expected for the ratio after the shifting, for example, if the motor generator is controlled to increase the rotational speed of the engine during a downshifting, then it can generate a torque that accelerates the vehicle, resulting in a feel of acceleration even during a downshifting. This can be a problem that gives also a sense of incongruity.